document when disk image for create_vps was made

.gitlab/ci_scripts/create_vps.sh

@@ -16,6 +16,7 @@ python3 -m openappstack $HOSTNAME create \
   --ssh-key-id $SSH_KEY_ID \
   --create-domain-records \
   --subdomain $SUBDOMAIN \
+  # Built with 386e3614 on 2021-07-08
   --disk-image-id '-8115' \
   --truncate-subdomain \
   --docker-mirror-server="${CI_DEPENDENCY_PROXY_SERVER}" \